At present, GPS plane control net is widely applied, but the GPS elevation is not used widely. The people expect that the GPS elevation surveying can take the place of the conventional leveling.

DEM (Digital elevation Model) is a data set of plane coordinate or longitude, latitude, and elevation of the regular square grid points in some region.

To manufacture besides the installation elevation conforming to the design requirement, it is very important to guarantee the level of the two directions of the plane.